Polysiloxane based elastomer compounds (silicones) are known for their interesting properties that can not be provided by any other polymer to that extent, such as very good hot air resistance and low temperature flexibility, “inorganic” backbone, chemical inertness etc. However, as the synthesis of polysiloxanes requires some significant efforts, silicones are relatively expensive. Crosslinking of polysiloxanes to elastomers is done at room temperature by support of metal, e.g., tin compound catalysts; at higher temperatures peroxides or noble metal compound catalysts are preferred, all these methods again contributing to the final compound or part price. Therefore some attempts have been taken to crosslink polysiloxanes by sulphur compounds, as it has been common in the organic rubber industry.
JP 11140316 mentions the possibility of crosslinking silicone by sulphur chloride, a very reactive agent that will cause side reactions and by-products which is not favoured in the industry. U.S. Pat. No. 4,657,965/EP 0183382 describes the use of significantly modified organic side groups of the polysiloxane to render them more accessible to sulphur radical attack and crosslinking, U.S. Pat. No. 4,617,347 claims a blend of silicone and organic rubbers which is said not to be directly curable by sulphur. The solution is said to be a modification of terminal vinyl groups of the silicone by again rather unfavoured chemistry (chlorothio-sulfonamides) to render them sulphur-compatible. These methods however, will lead to even more expenses in synthesis of the respective polysiloxanes.
U.S. Pat. No. 4,066,603/DE 2702046 and U.S. Pat. No. 4,151,156 both disclose the synthesis of mercapto-organo substituted siloxanes that would in principle also be accessible for sulphur, similar for JP 2107657 claiming amino-modification, but it can be expected that such siloxanes are both expensive and will no more preserve the typical silicone property profile. Ind. Eng. Chem. 1957, vol. 49, pp 49-54 mentions the crosslinking of vinyl substituted polysiloxanes by sulphur. However, what could be achieved as final property profile was poor even in comparison to the 1950s′ silicones typically bad mechanical levels. Additionally, the vinyl content necessary to obtain reasonable crosslinking is higher by an order of magnitude than usually supplied today. U.S. Pat. No. 2,877,211 describes sulphur containing curing agents for silicone resins, but not elastomers. KR 100405123 mentions the crosslinking of a natural rubber/organic rubber/silicone compound (containing vinyl groups) with sulphur, but without disclosing a reproducible or feasible method; however, peroxide and irradiation crosslinking are mentioned as other—preferred—methods; KR 100404076 is more precise and clearly mentions the parallel use of sulphur and peroxide to cure an organic rubber/silicone compound where it can be expected that the peroxide is responsible for crosslinking the polysiloxane.
Sulphur and sulphur compounds thus are very rarely used in silicone industry as they will intoxicate silicone crosslinking catalysts and/or negatively influence processing or final properties. As a consequence, the use of sulphur for manufacturing expanded polysiloxanes has only been disclosed when using the sulphur compound as modifier, as for example in U.S. Pat. No. 5,998,548, where a sulphur compound is used as an additive (i.e., inhibitor or retarder) to influence the balance of crosslinking and expansion in metal catalyzed crosslinked foam, and in GB 788598 and GB 847081, where sulphur is present as co-crosslinker in resins.
The use of sulphur compounds as curing agents for the manufacture of expanded silicone elastomer is not believed to have been disclosed yet. However, sulphur compound based crosslinking could be interesting especially for expanded materials as the manufacturer will have much more possibilities to influence the curing of the silicone than with peroxides or e.g., platinum catalysts, leading to improved modification of properties and lower densities, as crosslinking is always working versus the expansion and thus has to be optimally suppressed and controlled. The lowest achieved densities of heat cured expanded silicone elastomers are between 250 and 300 kg/m3 (e.g., see US 20080214688, using one of the chemical blowing agents with the highest theoretical gas potential, azodicarbonamide). For room temperature vulcanizing silicone rubbers the lowest reachable density is said to be between 150 and 200 kg/m3, however, these materials are among the most expensive silicones. Further, the level of expansion is limited by the curing system and its controllability, and both the room temperature curing as well as the high temperature crosslinking systems are too fast in one or the other way to efficiently use the gas formation potential of the used expansion agents.